1. Field of the Invention
The present invention relates to phase shift masks for lithographic projection apparatus, methods of manufacturing phase shift masks, and devices manufactured with phase shift masks according to the invention.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of apparatus. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interperted as encompassing various types of projection system, including refractive opties, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,769.
Alternating phase shift masks are used to increase the resolution of optical lithography systems. Alternating phase shift masks increase the resolution by introducing a 180° phase shift in the light transmitted between adjacent features on the mask.
FIG. 2 is a schematic illustration of an alternating phase shift mask 100 according to known construction. The mask 100 includes a glass or quartz layer 110 and a layer of opaque material, i.e. a hard mask 120. The hard mask 120 may be formed of, for example, chromium. The glass or quartz layer 100 includes portions which have been etched to define features 130 of the pattern. An area 140 of the glass or quartz layer 100 between the hard mask 120 defines an increased path length for the source radiation 200 and shifts the source radiation 200 passing through the area 140 between adjacent features 180° out of phase with the source radiation 200 passing through the etched portions that define the adjacent features 130. In order to produce the 180° phase shift, the features 130 are etched to a depth D equal to 0.5λ/(n−1), wherein λ is the wavelength of the source radiation 200 and n is the index of refraction of the glass or quartz layer 110.
It is difficult to control the etch rate and time of the glass or quartz layer 110 to the depth D. Variations in the material of the glass or quartz layer 110 cause variations in the depth D across the surface of the mask 100 and control over the etch rate and time must be accurately controlled to accurately produce the depth D. Variations in the depth D throughout the mask 100 cause variations in the phase shift between adjacent features so that the phase shift between adjacent features may be, for example, 175° or 185°. The variation in phase shift between adjacent features of the mask 100 results in decreased resolution and critical dimension uniformity of the mask 100.
FIG. 3 is a schematic illustration of another alternating phase shift mask 150 of known construction. The mask 150 includes a layer 160 of quartz or glass. The mask 150 includes etched portions that define features 131, 132 of the pattern. The feature 132 has a smaller critical dimension CD than the feature 131. The mask 150 also includes a hard mask 120 and an area 145 between the etched portions 131 and 132 that shifts the source radiation 200 passing through the area 180° out of phase with the source radiation 200 passing through the features 131, 132.
The difficulty of accurately controlling the etch rate and time also makes it difficult to form fine or small pattern features adjacent larger pattern features because features of different size etch at different rates. Small features requiring higher etch rates and lower etch times are etched to the desired depth before large features requiring lower etch rates and higher etch times.
Fine or small pattern features, i.e. features having a small CD, also tend to act as tunnels or fibers for the source radiation 200. As the source radiation 200 reflects off the sidewalls 133 of the feature 132 the boundary effect between the quartz or glass of the layer 160 and air decreases the phase shift of the source radiation 200 and reduces the resolution of the mask 150. This boundary effect is more pronounced in high NA systems, in particular those systems used in immersion lithography.
The benefits of phase shifting decrease with increasing feature size. Resolution improvement for larger features may be accomplished with attenuated phase shift masks. Resolution may be improved with phase shifts, for example, of 90°. Although they provide lower resolution and process latitudes than alternating phase shift masks, attenuated phase shift masks are simpler to design and fabricate than alternating phase shift masks.